Ever since 1950, when blood oxygenators made open heart surgery feasible, it has been possible to treat some forms of heart disease by replacing one of the patient's heart valves with a prosthetic valve. Early heart valve prostheses included ball-and-cage valves and disc-and-cage valves in which a ball or a disc was housed in a cage. One side of the cage provided an orifice through which blood flowed either into or out of the heart, depending on the valve being replaced. When blood flowed in a forward direction, the energy of the blood flow forced the ball or disc to the back of the cage allowing blood to flow through the valve. When blood attempted to flow in a reverse direction, or "regurgitate", the energy of the blood flow forced the ball or disc into the orifice in the valve and blocked the flow of blood.
A bi-leaflet valve comprised an annular valve body in which two opposed leaflet occluders were pivotally mounted. The occluders were typically substantially rigid, although some designs incorporated flexible leaflets, and moved between a closed position, in which the two leaflets were mated and blocked blood flow in the reverse direction, and an open position, in which the occluders were pivoted away from each other and did not block blood flow in the forward direction. The energy of blood flow caused the occluders to move between their open and closed positions.
A tri-leaflet valve comprised an annular valve body in which three flexible leaflets were mounted to a portion of the valve body, called a "stent," located at the circumference of the annulus. Some tri-leaflet valves used rigid leaflets. When blood flowed in the forward direction, the energy of the blood flow deflected the three leaflets away from the center of the annulus and allowed blood to flow through. When blood flowed in the reverse direction, the three leaflets engaged each other in a coaptive region, occluded the valve body annulus and prevented the flow of blood. The valve leaflets were made from tissue, such as specially treated porcine or bovine pericardial tissue, or, more recently, from a man-made material such as polyurethane or another biocompatible polymer.
Prosthetic heart valves should be reliable and durable because replacing a failed implanted valve is expensive and dangerous for the patient. Typically, the heart valve leaflets are the component most likely to fail. One of the factors that contributes to structural failure of heart valve leaflets is the level of stress leaflets experience in operation. Flexible membranes, such as heart valve leaflets, experience two kinds of stress. The first, called "membrane stress," is produced by the two-dimensional stretching of the membrane, and is linearly related to the pressure difference across the membrane (although the stress constants will vary due to curvature and thickness).
The second kind of stress, called "bending stress," is linearly related to the change in local curvature from the stress-free condition (although, again, the stress constants will vary according to the thickness of the membrane). The membrane stress and bending stress are superimposed to determine maximum stress. Lowering either type of stress will reduce maximum stress.